1. Field of the Invention
The present invention relates to the cleaning or purifying of contaminated water or soil. More particularly, this invention relates to a method both in-situ and ex-situ treatment for the immobilization of inorganic arsenic species, such as arsenates and arsenites using zero valent metals.
2. Brief Description of the Related Art
The present invention relates to the treatment of water or soils containing hazardous or undesirable compounds and more particularly relates to a system for the treatment of water or soils containing arsenic contaminants. Arsenic is a metaloid element that has been notorious for its toxicity. It is a group 5A nonmetal found in nature in the -3, 0, +3, and +5 oxidation states. Arsenic is found naturally as a main component of several minerals such as arsenopyrite, which is a white to steel-gray mineral found in crystalline rock. Over the years, arsenic has been used for a variety of purposes in the medical field, the cosmetic industry, and in agriculture. In the area of agriculture, arsenic-containing compositions have been used as insecticides, and are still used as desiccants, rodenticides, and herbicides. Arsenic also has been used in industrial applications involving the doping of solid state devices, as a laser material, in bronzing, and the like. Furthermore, arsenic can also be found in coal and coal combustion by-products.
A concern with the use of arsenic-containing compositions is their toxicity. Problems relating to contamination of water and soils with heavy metals such as arsenic has become increasingly evident in recent years. Heavy metals form poisonous compounds which, when taken into the human body, cause or are suspected to cause a variety of severe health problems including cancer, neurological impairment and birth defects. The use of arsenic as a component of herbicides has led to contamination at landfills and along railroad trails where the herbicides are applied. Arsenic contamination is also prevalent at gasoline transfer stations, chemical waste dump sites, areas around mining activities, and smelters, metal finishing/plating/electronics sites, wood treating sites, pharmaceutical manufacturing sites, and oil and solvent recycling sites. In addition, naturally occurring mineral deposits containing arsenic can contaminate drinking water supplies.
In response to the contamination of water and soils by heavy metals such as arsenic, the United States Environmental Protection Agency (EPA) and others have developed standards for the permissible level of heavy metals that may be present in drinking water and other types of water and soils. Various state and federal governmental bodies are responsible for promulgating specific criteria for remediation standards for arsenic. For example, the State of Connecticut's water quality and remediation standards for arsenic are a surface water protection criteria of 4 .mu.g/L; a ground water protection criteria and drinking water standards of 50 .mu.g/L; a GA pollutant mobility criteria of 50 .mu.g/L; a GB pollutant mobility criteria of 500 .mu.g/L; a residential direct exposure criteria of 10 mg/Kg; and industrial direct exposure criteria of 10 mg/Kg. Such increasingly stringent standards for heavy metal contamination highlights the need for effective and economical remediation methods.
Inorganic arsenic species exist in a variety of forms at contaminated sites. For example, inorganic arsenic species in contaminated industrial sites exist in the arsenate form (oxidation state=V), arsenite form (oxidation state=III), as arsenic sulfide (HArS.sub.2), elemental arsenic (As.sup.0) and arsine gas (AsH.sub.3) (oxidation state=III). The arsenate forms include H.sub.3 AsO.sub.4, H.sub.2 AsO.sub.4.sup.-, HAsO.sub.4.sup.-2 and AsO.sub.4.sup.-3. Arsenite forms include H.sub.3 AsO.sub.3, H.sub.2 AsO.sub.3.sup.-, HAsO.sup.-2, and AsO.sub.3.sup.-3. Arsenite (III) and arsenate (V) are the most common forms found in drinking water and waste water streams.
Systems for treating contaminated water and contaminated soil are known in the art. One of the well-known conventional systems for removing contaminants from water is to pass the contaminated water through a body of activated carbon. Activated carbon is a highly absorptive material, such that the dissolved contaminants are removed from the water and retained on the activated carbon. Over periods of time, the contaminant builds up on the activated carbon. After significant build up of the contaminant on the activated carbon, the carbon may be removed and disposed, flushed or otherwise treated (regenerated) to remove the contaminant. The drawback with such a system is that the contaminant still remains intact and hazardous.
Other existing systems for removing arsenic from drinking water include adsorption onto activated alumina within a fixed bed contractor; complexing arsenic with hydrous metallic floc, primarily aluminum and iron hydroxides or oxyhydroxides, in conventional water treatment plants; sieving the metal from water by membrane technologies using inverse osmosis; and electro-dynamic processes such as electrodialysis. Despite the existing systems, there remains a need for an economical, safe method to remove arsenic from water sources and from soil sites, such as landfills. In particular, there is a need for a system for in-situ immobilization of inorganic arsenic species, such that the arsenic species are broken down into harmless, or at least less hazardous, chemical substances.